In transport systems bus bunching, clumping, or platooning refers to a group of two or more transit vehicles along the same route, such as buses or trains, which are scheduled to be evenly spaced according to distance and/or time, but are running near the same location at the same time. This occurs when at least one of the vehicles is unable to keep to a planned schedule and therefore ends up in the same location as one or more other vehicles of the same route at the same time. The end result can be longer wait times for some passengers on routes that have shorter scheduled intervals.
Considering bus based transportation systems specifically, bus bunching can be caused by an inconsistent or uncharacteristic number of passengers needing to board or leave a bus at system bus stop. This may cause the bus currently at the bus stop to be delayed in the scheduled route, which in turn can cause the busses following the stopped bus to shorten the relative distance between the buses on the route. A delayed bus can also cause a larger relative distance between the stopped bus and the busses ahead of the stopped bus on the route.
When bus bunching occurs in a transit system, the system becomes inefficient for the service provider and for commuters. An accumulation of stop delays and other events on a bus route can result in bus bunching and cause prospective bus passengers to have extended wait times, or overcrowded buses. For example, if three buses are travelling exactly behind each other on the same route and direction, the two latter buses may be merely wasting fuel, while passengers just arriving at previously covered bus stops may have a long wait time. Bus bunching can cause an inefficient use of transportation system resources as some busses will be overcrowded with passengers, and others may end up underutilized and almost empty. Bus bunching can then result in the inefficient use of resources for the transit agency, for example fuel or personnel use, since one or more empty buses can be travelling at the same place and time.